Greener Catalysts for Greener Processes: Application of supercritical antisolvent precipitation for catalyst preparation

[ACSGCI]

​Contributed by Simon A. Kondrat, Postdoctoral Research Associate; Stuart H. Taylor, Professor of Physical Chemistry and Deputy Director of the Cardiff Catalysis Institute; and Graham J. Hutchings, Regius Professor of Physical Chemistry and Director of the Cardiff Catalysis Institute, Cardiff University

From the Principles of Green Chemistry, it can be observed that catalysis plays an important role in establishing green and sustainable processes by replacing stoichiometric reagents and improving atom efficiency and energy efficiency. Research in the field focuses on the development and understanding of catalytic materials for both new process technologies and well-established existing commercial processes. The industrially important methanol synthesis and low-temperature water-gas shift (LTS) reactions represent such established processes, where significant research into heterogeneous catalyst design and the understanding of structure-activity relationships is being pursued by the scientific community.

Cu/ZnO/Al₂O₃ is the commercial catalyst of choice for methanol synthesis and LTS reactions. The preparation method of the catalyst is known to be highly influential on its performance, with the conventional preparation method being based around the co-precipitation of metal nitrate solutions with a basic solution, usually sodium or ammonium carbonate. This co-precipitation process, if performed under optimal conditions, produces a copper zinc hydroxycarbonate catalyst precursor analogous to the mineral zincian malachite. Treatment of this hydroxycarbonate by calcination and then reduction produces the final active catalyst: highly dispersed copper nanoparticles interspersed with small zinc oxide crystallites.

While the co-precipitation method is well-established and commercially successful, it is not without a number of disadvantages. Firstly, the high concentration of metal nitrates and alkali salts results in a significant volume of water required to remove these impurities, which affects catalytic performance if not removed. The resulting waste water must then be processed at significant expense before it is discharged into the environment. A second consideration is that the aqueous co-precipitation technique does not exert ideal control of crystal growth or, more specifically, the recrystallization of the initial precipitates formed in the process. The initial precipitate in the co-precipitation process is an amorphous meta-stable phase that rapidly ages to malachite. Consequently, the instability of the amorphous phase formed initially by co-precipitation has prevented its investigation as an alternative catalyst precursor.

The team at the Cardiff Catalysis Institute (CCI) has recently developed a supercritical carbon dioxide anti-solvent (SAS) precipitation technique for catalyst synthesis. A supercritical fluid is a substance above its critical temperature and pressure, where distinct gas and liquid phases do not exist and the substance has intermediate properties between the two phases. The high solvating power combined with high diffusion rates and lack of surface tension results in the precipitation of high purity amorphous materials (for details, click here). This technology allowed the CCI to synthesize an amorphous copper zinc hydroxycarbonate phase, analogous to the extremely rare mineral georgeite, known to naturally exist in only three locations on Earth. It was proposed that this material is the same as that of the unstable initial precipitate formed by the conventional co-precipitation methodology.

As the SAS prepared material is dry from the point of precipitation, the CCI found that the zincian georgeite was highly stable, allowing our team to conduct in-depth characterization and use it as a precursor to make catalysts for methanol and water-gas shift for the first time. In addition, as carbon dioxide was used as the precipitating agent, no alkali metal salt is required, removing the necessity for post-preparation washing steps and potentially saving water. The SAS technique has the added benefit of easy recycling of precursor solvents as the simple depressurization of the carbon dioxide-solvent mixture allows for the separation of the two components.

It was found that the disordered nature of the zincian georgeite catalyst precursor prepared by SAS could be retained in the final catalyst by controlled heat treatment and then reduction. The catalyst has small, stable copper crystallites, and importantly, a high degree of interaction between the copper nanoparticles and the nanocrystalline zinc oxide. Combined with extremely low levels of catalyst poisons, such as sodium species, the unique microstructure of zincian-georgeite-derived catalysts provides exceptional catalytic performance for the methanol synthesis reaction and the low-temperature water-gas shift.

The supercritical antisolvent precipitation technique offers advantages for the preparation of catalysts by greener routes compared to some of the more traditional routes. Furthermore, it offers a powerful tool for catalyst discovery, as it enables researchers to access new materials with unique morphologies and microstructures that may not be readily accessible using more conventional synthesis routes. Against this background, the scope to apply supercritical preparation methods to catalysts has largely been unexplored, and it will provide a fruitful area of research that yields many new discoveries.

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